Focus detection apparatus and image pickup apparatus

ABSTRACT

A focus detection apparatus includes a determination unit configured to determine a degree of effect of noise included in a pair of parallax image signals, and an acquisition unit configured to acquire information about a phase difference between the pair of parallax images based on a calculation of correlation between the pair of parallax image signals. The acquisition unit selects a filter used to acquire the information about the phase difference from among a plurality of filters having different frequency characteristics based on a determination result of the determination unit, and outputs, as a focus detection result, the information about the phase difference acquired by the correlation calculation based on the pair of parallax image signals applied to the selected filter.

BACKGROUND Field of Invention

The present disclosure relates to a focus detection apparatus and animage pickup apparatus which use a phase difference detection method.

Description of Related Art

As a focus detection method based on a phase difference detectionmethod, conventionally, a light beam from a subject image is divided onan exit pupil surface and the divided light beams are respectivelyfocused on a pair of pixel columns in a two-dimensional image sensor.The focus detection is performed based on a phase difference betweenimage signals photoelectrically converted in the pair of pixel columns.

Japanese Patent Application Laid-open No. 2014-89260 discusses an imagepickup apparatus that performs the focus detection based on a phasedifference detection method using a two-dimensional image sensor inwhich microlenses are formed in respective pixels. In such an imagepickup apparatus, each pixel includes a photodiode A forphotoelectrically converting one image obtained by pupil division, aphotodiode B for photoelectrically converting the other image, and afloating diffusion area for temporarily holding electric charges fromthe photodiodes A and B. Electric charge transfer from the photodiodes Aand B to the floating diffusion area is controlled as follows: First,only the electric charges from the photodiode A are transferred andvoltage-converted to obtain one of the pupil-divided image signals (theimage signal is hereinafter referred to as an “A image signal”).Subsequently, the electric charges from the photodiode B are transferredto the floating diffusion area and voltage-converted, without resettingthe floating diffusion area to obtain an image signal before the pupildivision (hereinafter referred to as an “A+B image signal”).

The image signal corresponding to the photodiode B on which the otherpupil-divided image is incident (the image signal is hereinafterreferred to as a “B image signal”) is electrically obtained bysubtracting the A image signal from the A+B image signal. The A imagesignal and the B image signal which are obtained as described above as apair of image signals are parallax image signals. Accordingly, a focalposition of a subject can be detected by calculating the phasedifference between the A image signal and the B image signal by a knowncorrelation calculation technique. Further, the A+B image signal is usedfor generating an image captured by an image pickup apparatus. In thismanner, the image pickup apparatus discussed in Japanese PatentApplication Laid-open No. 2014-89260 is capable of acquiring three typesof image signals, i.e., the A image, the B image, and the A+B image, byexecuting a reading operation only twice. Accordingly, the speed ofreading operation is increased. In addition, since the A+B image signalis generated by mixing the electric charges in the floating diffusionarea, the image signal before the pupil division (A+B image signal) haslower noise than that obtained by a method of adding the A image signaland the B image signal after reading out the A image signal and the Bimage signal. However, appropriate correction of the effect of noiseincluded in the parallax image signals remains desirable.

SUMMARY

According to at least one embodiment, an image pickup apparatus includesa determination unit configured to determine a degree of effect of noiseincluded in a pair of parallax image signals, and an acquisition unitconfigured to acquire information about a phase difference between thepair of parallax image signals based on a calculation of correlationbetween the pair of parallax image signals. The acquisition unitselects, based on a determination result of the determination unit, afilter used to acquire the information about the phase difference fromamong a plurality of filters having different frequency characteristics,and outputs, as a focus detection result, the information about thephase difference acquired by the correlation calculation based on thepair of parallax image signals applied to the second filter.

Further features and advantage of the present disclosure will becomeapparent from the following description of exemplary embodiments withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a schematic configuration of adigital camera as an example of an image pickup apparatus according to afirst exemplary embodiment.

FIG. 2 is a graph illustrating a characteristic example of a digitalfilter for performing a process on a focus detection pixel columnaccording to the first exemplary embodiment.

FIGS. 3A and 3B are schematic diagrams each illustrating a configurationof an image sensor.

FIG. 4A illustrates a circuit configuration of a pixel of the imagesensor, and FIG. 4B illustrates a drive timing chart.

FIG. 5 illustrates a pattern subject used for describing an output ofnoise appearing in an image signal.

FIGS. 6B and 6C are graphs respectively illustrating image signals fromfirst and second focus detection pixel columns on the pattern subject,and FIG. 6A is a graph illustrating an addition signal of the first andsecond focus detection pixel columns.

FIGS. 7B and 7C are graphs respectively illustrating image signals fromfirst and second focus detection pixel columns when there is noisecontamination, and FIG. 7A is a graph illustrating an addition signal ofthe first and second focus detection pixel columns with reduced noisecontamination.

FIGS. 8A, 8B, and 8C are graphs each illustrating a correlation betweenfirst and second sub-pixel columns for each phase shift amount.

FIG. 9 is a flowchart illustrating a focus detection operation of thedigital camera according to the first exemplary embodiment.

FIG. 10 is a flowchart illustrating a process of determining a degree ofeffect of noise according to the first exemplary embodiment.

FIG. 11 is a flowchart illustrating a process of determining a degree ofeffect of noise according to a second exemplary embodiment.

FIG. 12 is a graph illustrating a relationship between a sharpness of animage signal and a defocus amount according to the second exemplaryembodiment.

DESCRIPTION OF THE EMBODIMENTS

Exemplary embodiments will be described in detail below with referenceto the drawings. In the exemplary embodiments, a focus detectionapparatus according to the present disclosure is applied to alens-interchangeable digital single-lens reflex (SLR) camera. However,the focus detection apparatus according to the present disclosure can beapplied to any electronic imaging processing device equipped with orconnected to an image sensor capable of generating signals used forfocus detection based on a phase difference detection method. Examplesof electronic imaging devices include non-lens-changeable type digitalcameras, video cameras, cellular phones equipped with a camera, personalcomputers processing images acquired from a camera, game consoles, andeven household electrical appliances equipped with or connected to acamera.

The focus detection apparatus according to an exemplary embodimentdescribed below is a focus detection apparatus that performs focusdetection based on an image plane phase difference method, anddetermines a degree of effect of noise included in a pair of parallaximage signals. In addition, the focus detection apparatus acquiresinformation about an image shift amount obtained by a correlationcalculation based on the pair of parallax image signals. Band-passfilter processing selected based on the determination result isperformed on the parallax image signals. For example, if it isdetermined that the effect of noise is large, a band-pass filter that isless susceptible to noise and transmits relatively low frequency bandsignals is selected. On the other hand, if it is determined that theeffect of noise is small, a band-pass filter capable of performing focusdetection with a high accuracy and transmitting relatively highfrequency band signals is selected. Thus, the focus detection in whichthe effect of noise is reduced can be performed.

Configuration of Digital Camera System

FIG. 1 is a block diagram illustrating a schematic configuration of adigital camera system as an example of an image pickup apparatusaccording to a first exemplary embodiment of the present disclosure.

Referring to FIG. 1, the digital camera system includes a lens unit 100and a camera unit 200. The lens unit 100 is detachably attached to thecamera unit 200 through a lens mounting mechanism of a mounting unit(not illustrated). The mounting unit is provided with an electriccontact unit 108. The electric contact unit 108 includes a terminal fora communication bus line, which allows the lens unit 100 and the cameraunit 200 to communicate with each other.

The lens unit 100 includes a lens group 101 and a diaphragm 102 forcontrolling incident light rays. The lens group 101 constitutes an imagepickup optical system and includes a focus lens and a zoom lens. Thelens unit 100 also includes a lens drive unit 103 that includes a drivesystem and a drive system control unit. The drive system includes astepping motor for zooming or focusing the lens group 101. The lensdrive unit 103 and the drive system constitute a focus adjustment unit.The lens unit 100 also includes a diaphragm control unit 104 and anoptical information recording unit 106. The diaphragm, control unit 104controls the aperture of the diaphragm 102. The optical informationrecording unit 106 records various optical design values for zooming orfocusing of the lens group 101 and values for the diaphragm. The lensdrive unit 103, the diaphragm control unit 104, and the opticalinformation recording unit 106 are each connected to a lens controller105 which includes a central processing unit (CPU) for controlling theoverall operation of the lens unit 100. The lens unit 100 also includesa lens position detection unit 107 that detects positional informationof a lens, for example, by acquiring a phase wave form of the steppingmotor included in the lens drive unit 103.

The camera unit 200 communicates with the lens unit 100 via the electriccontact unit 108 to transmit a control request for controlling zoomingor focusing of the lens group 101 and the aperture of the diaphragm 102,and receive control results. The camera unit 200 includes an operatingswitch 214 for inputting an operation into the camera unit 200. Theoperating switch 214 includes two-stage-stroke-type switches. Afirst-stage switch (SW1) is a switch for starting an image pickuppreparation operation such as photometry and focus detection usingcaptured image signals. A second-stage switch (SW2) is a switch forcausing an image pickup unit 213 to start an image pickup operation suchas electric charge accumulation and electric charge reading, to acquirea still image.

The image pickup unit 213 includes an image sensor, an A/D converter,and a processor that performs a development calculation. The imagesensor includes a plurality of pixels each including a photoelectricconversion unit. The A/D converter converts an electrical signal outputfrom the image sensor into an image signal as digital data Incident raysare guided to the image sensor through the lens group 101 and thediaphragm 102. The image pickup unit 213 performs a developmentcalculation by photoelectrically converting an incident subject image toobtain captured image data. As the image sensor, a charge-coupled device(CCD) image sensor or a complementary metal oxide semiconductor (CMOS)image sensor can be used. The A/D converter may be incorporated in theimage sensor. A phrase “a pixel outputs an electrical signalcorresponding to an image signal” used herein may be also simplyexpressed as “a pixel outputs an image signal” in the presentspecification. At least some of the pixels included in the image sensorcan output the image signal to be used for focus detection. The imagesignal for focus detection obtained by the image sensor is temporarilystored in a memory 216 which is connected to a camera controller 215. Apair of parallax image signals temporarily stored in the memory 216 (thepair of parallax image signals is hereinafter also referred to simply asa pair of image signals) is sent to a pixel addition unit 217 which isconnected to the camera controller 215.

The pixel addition unit 217 adds the image signal obtained from thepositionally corresponding pixel to the pair of image signals until thenumber of times counted by an addition counter 218 reaches apredetermined number of times. The pair of image signals to which theimage signal is added is sent to a correlation calculation unit 219. Thecorrelation calculation unit 219 is a correlation amount acquisitionunit connected to the camera controller 215, and calculates acorrelation amount (represented by a difference between the pair ofimage signals) for each phase shift amount of the pair of image signalsby carrying out a correlation calculation. A correlation amount additionunit 220 connected to the camera controller 215 adds the calculatedcorrelation amounts until the number of times counted by the additioncounter 218 reaches the predetermined number of times. The addedcorrelation amounts are sent to a phase difference detection unit 221 toobtain a phase difference which shows the highest correlation. As it isunderstood by those skilled in the art of signal processing, the term“correlation” refers to a measure of similarity of two signals as afunction of a variable (e.g., the displacement of one signal relative tothe other). Accordingly, when the term “phase difference” is used in thepresent disclosure and specification, it indicates a phase differencewith a highest correlation (a phase difference indicating a minimumvalue of a correlation amount) between two images constituting theparallax image signals unless otherwise noted.

A defocus amount acquisition unit 222 acquires a defocus amount by aknown method based on the phase difference acquired by the phasedifference detection unit 221 and optical characteristics of the lensunit 100. The acquired defocus amount is sent to a defocus amountaddition unit 223, and the defocus amount is added until the number oftimes counted by the addition counter 218 reaches the predeterminednumber of times.

The camera controller 215 transmits and receives control information toand from the lens controller 105 via the electric contact unit 108, andcontrols the focal position of the lens group 101 based on the defocusamount calculated by the defocus amount acquisition unit 222 or thedefocus amount addition unit 223.

The digital camera according to the present exemplary embodimentincludes a display unit 224 and an operation unit 225. The display unit224 displays a subject image captured by the image pickup unit 213 andvarious operation states. The operation unit 225 switches the operationof the image pickup unit 213 to a live view mode or a moving-imagerecording mode. Still images or moving images captured by the imagepickup unit 213 are recorded on a recording unit 226 in a predetermineddata format.

The camera unit 200 also includes a pixel subtraction unit 227 and aband-pass filtering (BPF) processing unit 228 that performsone-dimensional BPF on the image signals. The pixel subtraction unit 227receives two image signals from the image pickup unit 213 and subtractsone image signal from the other image signal. The BPF processing unit228 includes a first filter that transmits predetermined spatialfrequency characteristics, and a second filter showing a frequency bandlower than the first filter as its characteristics. FIG. 2 illustratesan example of spatial frequency characteristics of the first and secondfilters.

In FIG. 2, the spatial frequency characteristics of the first filter arerepresented by a solid line, and the spatial frequency characteristicsof the second filter are represented by a dashed line. “Ny” in FIG. 2represents Nyquist frequencies of a pair of focus detection pixelcolumns. The first filter has spatial frequency characteristics having amaximum amplitude at a frequency f1. The second filter has spatialfrequencies having a maximum amplitude at a frequency f2 which is lowerthan the frequency f1. Accordingly, it can be seen that the secondfilter is a band-pass filter that transmits signals in a frequency rangelower than the first filter. In addition, the BPF processing unit 228performs a normalization operation on the maximum amplitudes of thefirst and second filters after the filter operation is performed.Therefore, the value of a maximum amplitude of the first and secondfilters is both “1”.

The camera unit 200 also includes a storage area for temporarily storingonly two sets of correlation amounts for each phase differencecalculated by the correlation calculation unit 219. Further, the cameraunit 200 includes a correlation amount difference calculation unit 229that calculates a correlation amount difference in any phase shiftamount between two sets of correlation amounts. The correlation amountdifference calculation unit 229, together with the camera controller215, functions as a determination unit that determines a degree ofeffect of noise included in a pair of parallax image signals.

Next, the configuration of the image sensor included in the image pickupunit 213 will be described with reference to FIGS. 3A and 3B and FIGS.4A and 4B.

FIG. 3A is a schematic diagram illustrating a light receiving surface ofthe image sensor as viewed from an includes a first sub-pixel S1 and asecond sub-pixel S2. A pair of rays divided on the exit pupil surface isrespectively incident on the first sub-pixel S1 and the second sub-pixelS2. A light beam, passing through a first pupil area 710 of an exitpupil 700 is incident on the first sub-pixel S1, and a light beampassing through a second pupil area 720 of the exit pupil is incident onthe second sub-pixel S2. A microlens ML for focusing light is disposedon the front surface of each of the first sub-pixel S1 and the secondsub-pixel S2. Each pixel includes a set of the first sub-pixel S1, thesecond sub-pixel S2, and the microlens ML. h pixels in a horizontaldirection and v pixels in a vertical direction are arranged on the lightreceiving surface of the image sensor. Image signals from the first andsecond sub-pixels S1 and S2 can be used for focus detection, and animage signal from a unit pixel can be used for image capturing. Thefirst sub-pixel and the second sub-pixel that form the same pixel may bereferred to as a pair of sub-pixels. Light beams passing through thesame microlens ML are respectively incident on the pair of sub-pixels.

FIG. 3B is a sectional view of each pixel illustrated in FIG. 3A. Themicrolens ML, a smoothing layer 701, which constitutes a plane forarranging the microlens ML, light shielding layers 702 (702 a, 702 b),and first and second photodiodes 703 a and 703 b are formed in thisorder from the light incident side (upper side in FIG. 3B). The lightbeam A passing through the first pupil area 710 of the exit pupil 700 isincident on the first photodiode 703 a, and the light beam B passingthrough the second pupil area 720 is incident on the second photodiode703 b. The light shielding layer 702 is located so as to preventunwanted oblique rays from entering the photoelectric conversion area703 a of the first focus detection pixel S1 and the photoelectricconversion area 703 b of the second focus detection pixel S2. With sucha pixel configuration, the pupil of the image pickup optical system issymmetrically divided and image signals respectively corresponding tothe divided light beams can be obtained.

Next, a circuit configuration of each pixel of the image sensor will bedescribed with reference to FIG. 4A. A pixel 300 includes a firstphotodiode 301A, a second photodiode 301B, a first transfer switch 302A,a second transfer switch 302B, and a floating diffusion area 303. Theunit pixel 300 also includes an amplification unit 304, a reset switch305, and a selection switch 306. The pixel 300 also includes a commonpower supply VDD 308. The first photodiode 301A and the first transferswitch 302A constitute the first sub-pixel S1, and the second photodiode301B and the second transfer switch 302B constitute the second sub-pixelS2.

The first and second photodiodes 301A and 301B photoelectrically convertthe light rays, which have passed through the same microlens ML and arerespectively incident on the first and second photodiodes 301A and 301B,into corresponding electric signals. The first and second transferswitches 302A and 302B control the electric charges generated by thefirst and second photodiodes 301A and 301B to be selectively transferredto the common floating diffusion area 303. The first and second transferswitches 302A and 302B are controlled by first and second transfer pulsesignals PTXA and PTXB, respectively. The floating diffusion area 303temporarily holds the electric charges transferred from the first andsecond photodiodes 301A and 301B, and converts the electric charges itholds, into a voltage signal. The amplification unit 304, which includesa source follower MOS transistor, amplifies the voltage signal based onthe electric charges held in the floating diffusion area 303, andoutputs the amplified voltage signal as a pixel signal. The reset switch304 resets the potential of the floating diffusion area 303 to areference potential VDD. The reset switch 305 is controlled by a resetpulse signal PRES. The selection switch 306 controls the output of thepixel signal from the amplification unit 304 to a vertical output line307. The selection switch 306 is controlled by a vertical selectionpulse signal PSEL.

Next, an operation for driving the circuit of the unit pixel 300illustrated in FIG. 4A will be described with reference to a drivetiming chart illustrated in FIG. 4B. First, the first and secondphotodiodes 301A and 301B are reset during a period from time t1 to timet2. At the time t2, the first and second photodiodes 301A and 301B startaccumulating electric charges. After the electric charges areaccumulated for a necessary period of time based on a desired exposureamount, at time t3, the selection switch 306 is turned on. At time t4,the reset state of the floating diffusion area 303 is released. During aperiod from time t5 to time t6, the first transfer switch 302A is turnedon to transfer the electric charges of the first photodiode 301A to thefloating diffusion area 303. At the time t6, the electric chargeaccumulated in the first photodiode 301A are read out to the floatingdiffusion area 303. When the electric charges are read out, a voltagesignal corresponding to a change in potential is output, as a firstfocus detection pixel signal, to the vertical output line 307 throughthe amplification unit 304 and the selection switch 306. During a periodfrom time t7 to time t8, the first and second transfer switches 302A and302B are turned on. As a result, the electric charges of the first andsecond photodiodes 301A and 301B are simultaneously transferred to thefloating diffusion area 303. At the time t8, the transferred electriccharges are read out to the floating diffusion area 303. When theelectric charges are read out, a voltage signal corresponding to achange in potential is output, as an addition signal of the first andsecond focus detection pixel signal, to the vertical output line 307through the amplification unit 304 and the selection switch 306. At timet9, the floating diffusion area 303 is reset.

The above-described operations are sequentially carried out for eachunit pixel, and then the operation of reading out the image signalacquired from the pixel column of the first sub-pixel, and the additionsignal of the image signals acquired from the pixel columns of the firstand second sub-pixels is completed.

The image signal acquired from the pixel column of the first sub-pixel,and the addition signal of the image signals acquired from the pixelcolumns of the first and second sub-pixels, which are read out asdescribed above, are input to the pixel subtraction unit 227, and theimage signal acquired from the pixel column of the second sub-pixel iselectrically generated. In the present specification, the image signalacquired from the pixel column of the first sub-pixel is referred to asan A image signal, the image signal acquired from the pixel column ofthe second sub-pixel is referred to as a B image signal, and the imagesignals acquired from the pixel columns of the first and secondsub-pixel are referred to as an A+B image signal. The B image signal isdeemed to be obtained from the second sub-pixel or the second photodiode301B even in a case where the B image signal is in effect obtainedindirectly without reading out singly the B image signal, as describedin the present exemplary embodiment.

Relationship Between a Phase Difference and a Defocus Amount

A phase difference (also referred to as an image shift amount) betweentwo images, which constitute a pair of parallax image signals and areacquired by a correlation calculation, and a defocus amount have aproportional relation, and the defocus amount monotonously increases asthe phase difference increases. The direction of the phase differencecorresponds to the direction of the defocus amount (closest distanceside/infinite distance side). The term “a phase difference between apair of parallax image signals” described herein refers to a phasedifference when the correlation between a pair of parallax image signalsis the highest (i.e., the correlation amount takes a minimum value)unless otherwise noted, in the present disclosure and specification. Thephase difference can be converted into a defocus amount by a knownmethod. In addition, a distance to a subject can be calculated based onthe defocus amount and a magnification relation of the lens group 101.

Effects of Noise Contamination in One of the Image Signals Obtained byPupil Division, on the Other Image Signals

Next, the image signals and the correlation amount when there is noisecontamination in the first or second sub-pixel column will be described.

FIG. 5 illustrates a pattern subject 1300 represented by a simplegraphic pattern for ease of explanation. The pattern subject 1300includes a gray portion 1301 and a white portion 1302. FIG. 5illustrates a pixel column area (so-called focus detection area) 1303which is used for focus detection and noted for the sake of description.

FIG. 6B is a graph illustrating the A image signal for the patternsubject 1300 illustrated in FIG. 5. FIG. 6C is a graph illustrating theB image signal for the pattern subject 1300. FIG. 6A is a graphillustrating the A+B image signal for the pattern subject 1300. Assumethat there is no phase difference between the A image signal and the Bimage signal in an in-focus state of the lens group 101.

FIG. 6A is a graph illustrating the A+B image signal as the additionsignal. An image signal output of the low-brightness gray portion 1300in intervals between X-coordinates, from X₀ to X₁ and from X₂ to X₃ inthe horizontal direction, is represented by Y₂. An image signal outputof the high-brightness white portion 1302 in an interval from X₁ to X₂is represented by Y₄. Even in a state where there is no incident lightamount, a constant signal value is added due to the effect of a darkcurrent generated in the image pickup unit 213, and thus the signallevel is represented by OB.

FIG. 6B is a graph illustrating the A image signal, and FIG. 6C is agraph illustrating the B image signal. In FIGS. 6B and 6C, an imagesignal output of the low-brightness gray portion 1301 in intervalsbetween X-coordinates, from X₀ to X₁ and from X₂ to X₃, in thehorizontal direction is represented by Y₁. An image signal output of thehigh-brightness white portion 1302 in an interval between X-coordinates,from X₁ to X₂, is represented by Y₃. The image signal output Y₁ is anintermediate value between the image signal output Y₂ and OB. The imagesignal output Y₃ is an intermediate value between the image signaloutput Y₄ and OB.

FIG. 7C is a graph illustrating the B image signal, when contaminationsby independent noises occur in the A image signal illustrated in FIG. 6Band the A+B image signal illustrated in FIG. 6A.

In the image pickup unit 213 according to the present exemplaryembodiment, timing for voltage-converting only the electric charges ofthe first sub-pixel column and timing for voltage-converting theelectric charges of the addition signal of the first and secondsub-pixel columns are different in the floating diffusion area 303.Accordingly, noise added to the electric charges of the first sub-pixelcolumn does not necessarily match noise added to the electric charges ofthe addition signal. For this reason, FIGS. 7A to 7C illustrate a casewhere there is the contamination by independent noise in the respectivesignals.

Contaminating noise in the A+B image signal is schematically representedby noise ΔN_(AB) (shown in FIG. 7A). Contaminating noise in the A imagesignal is schematically represented by noise ΔN_(A) (shown in FIG. 7B).Contaminating noise in the B image signal is schematically representedby noise ΔN_(B) (shown in FIG. 7C). In this case, the B image signal(FIG. 7C) is obtained by electrically subtracting the A image signal(FIG. 7B) from the A+B image signal (FIG. 7A). Accordingly, the B imagesignal (B+ΔN_(B)) is a signal represented by the following formula 1.B+ΔN _(B)=(A+B+ΔN _(AB))−(A+ΔN _(A))=B+ΔN _(AB) −ΔN _(A) hence, N _(B)=ΔN _(AB) −ΔN _(A)   (Formula 1)

A: a signal based on electric charges generated by the first photodiode(a signal obtained by eliminating the noise from the A image signal)

B: a signal based on electric charges generated by the second photodiode(a signal obtained by eliminating the noise from the B image signal)

ΔN_(AB): noise superimposed on the addition signal

ΔN_(A): noise superimposed on the A image signal

ΔN_(B): noise superimposed on the B image signal

As can be seen from a comparison between FIG. 7B and FIG. 7C, the noiseΔN_(A) and the noise ΔN_(B) are generated such that positive andnegative directions of the signal output level are reversed and thecorrelation between the image signals is lowered as compared withconfigurations of FIGS. 6B and 6C in which there is no noisecontamination.

FIGS. 8A, 8B, to 8C each illustrate a correlation curve obtained byperforming a correlation calculation on a pair of parallax imagesignals. FIG. 8A illustrates a correlation curve of each of the A imagesignal and the B image signal illustrated in FIGS. 6A to 6C. FIG. 8A isa graph in which the horizontal axis represents a phase shift amount andthe vertical axis represents a correlation amount (a difference betweentwo images). In FIG. 8A, the lens group 101 is in an in-focus state, andthus the correlation amount is a minimum value of C₁ when the phaseshift amount is zero (there is no difference between the two images).

FIG. 8B is a correlation curve of each of the A image signal and the Bimage signal illustrated in FIGS. 7A to 7C. In FIG. 8B, since there iscontamination with the noise ΔN_(A) and the noise ΔN_(B) in the A imagesignal and the B image signal, the correlation amount increases(correlation is above C₁) in a range of values ±P of the phase shiftamount across a value zero. The correlation amount is C₂ when the phaseshift amount has a value of ±P. However, at the position where the phaseshift amount is zero, the correlation amount is increased by an amountequal to ΔC_(N) above C₂ and reaches a correlation value C₃ . If such acorrelation amount is obtained, although it has to be determined thatthe correlation is the highest when the phase shift amount is zero,actually it is determined that the correlation is the highest when thephase shift amount is −P or +P, so that an error occurs. Occurrence ofsuch an error in the correlation amount may cause a deterioration inaccuracy of focus detection and/or occurrence of “hunting” during focusdetection.

FIG. 8C is a graph illustrating a difference between the correlationamounts illustrated in FIGS. 8A and 8B in the range of the phase shiftamount of −P to +P and the vicinity thereof. When the difference betweenthe correlation amounts is calculated, a local maximum is obtained at aphase shift amount of 0 as a result of increase in the correlationamount due to the noise contamination. When the amount of the noisecontamination is increased, the correlation deteriorates and thecorrelation amount at the position corresponding to the phase shiftamount of 0, i.e., the value C₃, increases. Accordingly, the correlationamount difference ΔC_(N) caused by a difference between the presence andabsence of noise contamination is increased. Accordingly, it isadvantageous to determine the difference in correlation amount caused bya difference between the presence and absence of noise contamination inthe parallax image signals.

Operation of Digital Camera System

FIG. 9 is a flowchart illustrating a focus detection operation of thedigital camera system according to the present exemplary embodiment.This operation is implemented in such a manner that the cameracontroller 215 controls each unit.

In the digital camera system according to the present exemplaryembodiment, after power-on, the operation mode is automatically set tothe live view mode, and the A image signal and the A+B image signal aregenerated while images are continuously captured by the image pickupunit 213. In addition, the subject image based on the A+B image signalis displayed on the display unit 224.

First, the first-stage switch SW1 of the operating switch 214 is pressedand a focus detection instruction is issued to start the focus detectionoperation.

In step S1901, the A image signal and the B image signal are acquired.In this step, the A image signal and the A+B image signal aretransferred from the image pickup unit 213 to the pixel subtraction unit227. The A image signal and the B image signal may also be referred toas two images. The pixel subtraction unit 227 electrically subtracts theA image signal from the A+B image signal, thereby acquiring the B imagesignal. Thus, the A image signal and the B image signal which areindependent from each other are acquired. After the acquisition, theprocessing proceeds to step S1902.

In step S1902, correction processing for suppressing various types ofsignal level variations such as a decrease in marginal illuminationcaused by a black level correction or the image pickup optical system,is performed on the two images independently. Variations in signallevels are caused because a ray from a subject image is divided on theexit pupil surface and rays of different angles are respectivelyincident on the pixel columns of the first sub-pixel S1 and the secondsub-pixel S2. After the correction processing, the processing proceedsto step S1903.

In step S1903, the degree of effect of noise described above withreference to FIGS. 7A to 7C and FIGS. 8A to 8C on the correctioncalculation is determined. The determination processing will bedescribed in detail with reference to FIG. 10.

In FIG. 10, at step S2001, the BPF processing unit 228 performs firstfilter processing on a pair of focus detection pixel columns, andoutputs the processing result to the correlation calculation unit 219.After the output, the processing proceeds to step S2002.

In step S2002, the correlation calculation unit 219 performs acorrelation calculation using the pair of focus detection pixel columnsthat has been subjected to the filter processing and acquired in stepS2001, and calculates a correlation amount (first correlation amount)for each phase difference. The calculated first correlation amount isoutput to the correlation amount difference calculation unit 229. Afterthe output, the processing proceeds to step S2003.

In step S2003, the BPF processing unit 228 performs second filterprocessing showing a frequency characteristics lower than the firstfilter on the pair of focus detection pixel columns, and outputs theprocessing result to the correlation calculation unit 219. After theoutput, the processing proceeds to step S2004.

In step S2004, the correlation calculation unit 219 performs acorrelation calculation on the pair of focus detection pixel columnsthat has been subjected to the filter processing and acquired in stepS2003, and calculates a correlation amount (second correlation amount)for each phase difference. The calculated second correlation amount isoutput to the correlation amount difference calculation unit 229. Afterthe second correlation amount is output, the processing proceeds to stepS2005. Since the second correlation amount is calculated by acorrelation calculation using signals in a frequency range lower thanthe first correlation amount, the correlation amount is obtained inwhich the effect of noise is lower than the first correlation amount.

In step S2005, the correlation amount difference calculation unit 229calculates a correlation amount difference between the first and secondcorrelation amounts at the phase difference of zero, and determineswhether the calculation result is equal to or greater than a firstthreshold. In addition, the correlation amount difference calculationunit 229 calculates a difference between the first correlation amountand the second correlation amount in a predetermined phase differenceshift amount range including the phase difference of zero, anddetermines whether a local maximum can be obtained at the phasedifference of zero. If the correlation amount difference at the phasedifference of zero is equal to or greater than the first threshold andis the local maximum (YES in step S2005), the processing proceeds tostep S2006. If not (NO in step S2005), the processing proceeds to stepS2007.

In step S2006, as described above with reference to FIGS. 7A to 7C andFIGS. 8A to 8C, the camera controller 215 determines that the degree ofeffect of correlation noise appearing in the vicinity of the positioncorresponding to the phase difference of zero is equal to or higher thana predetermined degree and the correlation between two images is largelydecreased, and the sub-routine operation is terminated.

On the other hand, in step S2007, the camera controller 215 determinesthat the degree of effect of correlation noise appearing in the vicinityof the position corresponding to the phase difference of zero is lowerthan the predetermined degree and a decrease in the correlation betweentwo images is small. Thus, the sub-routine operation is terminated.

Referring again to the flowchart of FIG. 9, in step S1904, the cameracontroller 215 determines whether a condition for taking acountermeasure against noise is met on the basis of the determinationresult obtained in step S1903. In step S1903, if the degree of effect ofnoise is equal to or higher than a predetermined degree (YES in stepS2005), the processing proceeds to step S1905. On the other hand, instep S1903, if it is determined that the degree of effect of noise islower than the predetermined degree (NO in step S2005), the processingproceeds to step S1906.

In step S1905, the phase difference detection unit 221 acquires a phasedifference showing the highest correlation amount between two imageswhich constitute a pair of parallax image signals based on the secondcorrelation amount in order to reduce the effect of noise in focusdetection. The second correlation amount is calculated by the secondfilter showing spatial frequency characteristics of a lower range thanthat of the first filter. After the phase difference is acquired, theprocessing proceeds to step S1907.

When the effect of noise on the focus detection is small, after thephase difference is acquired, the processing proceeds to step S1906.Therefore, in step S1906, a phase difference having the highestcorrelation amount between two images constituting a pair of parallaximage signals is acquired based on the first correlation amountcalculated by the first filter showing spatial frequency characteristicsof a higher range in order to increase the accuracy of detecting thephase difference while holding edge components of the subject image asmuch as possible. After the acquisition of the phase difference, theprocessing proceeds to step S1907. More specifically, in steps S1904 toS1906, a band-pass filter used to acquire the phase difference isselected from the first and second filters depending on thedetermination result as to the degree of effect of noise in step S1903.

In step S1907, the defocus amount acquisition unit 222 acquires adefocus amount by a known method based on the phase difference acquiredin step S1905 or S1906, and the optical characteristics of the lens unit100. The acquired defocus amount is sent to the defocus amount additionunit 223, and the defocus amount is added a predetermined number oftimes while the number of times is counted by the addition counter 218.Addition processing is performed to suppress variations in phasedifference detection. After the addition, the processing proceeds tostep S1908.

In step S1908, the focal position of the lens group 101 is controlledbased on the defocus amount that is calculated by the defocus amountaddition unit 223 and output from the focus detection apparatus, therebyperforming a focus adjustment.

The above-described operations enable determination of the degree ofeffect of correlation noise on the focus detection. In addition, anappropriate filter using the determination result is selected, so thatthe focus detection can be performed in which the effect of correlationnoise is reduced.

Modified Example of First Exemplary Embodiment

A modified example for step S1903 will now be described. According tothe first exemplary embodiment, in step S1903, the degree of effect ofnoise is determined based on the magnitude of the difference between thefirst correlation amount and the second correlation amount and based ona position of the local maximum by using the first and second filtershaving different spatial frequency characteristics. However, variousinformation may be added as information based on which the degree ofeffect of noise is determined.

For example, an intensity of a pair of parallax image signals may beadded. Specifically, a determination as to whether a maximum value or anaverage value of image signals of at least one of two imagesconstituting the pair of parallax image signals is equal to or smallerthan a second threshold can be added. The determination may be added tothe determination in step S2005. It may be determined that the effect ofnoise is large only when the correction amount difference at theposition corresponding to the phase shift amount of zero is equal to orlarger than the first threshold and is a local maximum, and theintensity of the pair of parallax image signals is equal to or less thanthe second threshold. The addition of such an operation can reduce thepossibility of an erroneous determination that the effect of noise islarge, when the difference between the first and second correlationamounts increases due to entering of a subject having a high frequencycharacteristic, under a condition that the image signal level is highand the degree of effect of noise is relatively low.

Instead of the intensity of the pair of parallax image signals, anintensity of an image signal for recording (a signal based on the A+Bimage signal) may be used as information based on which the degree ofeffect of noise is determined. The captured image signals acquired bythe image sensor, such as the pair of parallax image signals and theimage signal for recording, are referred to as captured image signals,and a signal based on the addition signal is referred to as an imagepickup signal in the present specification and disclosure.

An exposure value (EV) at an apex value may be acquired based on anelectric charge accumulation time of the image sensor included in theimage pickup unit 213 and a diaphragm value of the diaphragm 102, and itmay be determined that the effect of noise is large only when the EV isequal to or less than a third threshold.

In addition, it may be determined that the effect of noise is large onlywhen ISO is equal to or greater than a fourth threshold.

Next, a modified example for step S2005 will be described. According tothe first exemplary embodiment, in step S2005, it is determined whetherthe correlation amount difference is equal to or greater than apredetermined threshold at the phase difference of zero. However,reverse waveform noise such as ΔC_(N) appears not only at the positioncorresponding to the phase difference of zero, but is distributed in arange of phase shift amounts of −P to +P about the phase difference ofzero. For this reason, instead of calculating the difference between thefirst and second correlation amounts at the phase difference of zero,the correlation amount difference may be calculated in a predeterminedphase shift amount range about the phase difference of zero. Theoperation as described above enables more accurate determination as towhether the effect of noise is large. In addition, a correlation amountdifference in the vicinity of the phase difference of zero can be usedinstead of the correlation amount difference at the phase difference orzero.

Further, in first exemplary embodiment, the phase difference is acquiredin step S1905 or S1906 based on the correlation amount calculated instep S1903, but instead the correlation calculation may be carried outagain in step S1905 or S1906. While with this operation, the amount ofoperation increases, the phase difference can be acquired using a filterdifferent from the first and second filters used in step S1903.Therefore, filters suitable for detection of noise (first and secondfilters) and filters suitable for acquiring the phase difference(filters used for steps S1905 and S1906, which are referred to as thirdand fourth filters) can be selected. In addition, when a plurality ofthresholds for the difference between the first and second correlationamounts is set, the degree of effect of noise can be evaluated at threeor more stages to select filters to be used to acquire the phasedifference from among three or more band-pass filters depending on theevaluation result.

In step S1907, the defocus amount is acquired from the phase differencebetween two images, but instead a drive amount (a migration length, thenumber of pulses, etc.) necessary for causing a lens to directly move toan in-focus position may be acquired from the phase difference.Information acquired from the phase difference, such as a phasedifference, a defocus amount, and a drive amount necessary for causingthe lens to move to the in-focus position, is referred to as informationabout the phase difference.

A second exemplary embodiment will be described below with reference toFIG. 11. In the first exemplary embodiment, the degree of effect ofnoise is determined using the difference between the first and secondcorrelation amounts, and the phase difference is acquired using aband-pass filter having relatively lower frequency characteristics whenit is determined that the effect of noise is large.

The second exemplary embodiment differs from the first exemplaryembodiment in that the effect of noise is determined based oninformation about a contrast of a subject and information about noisecontamination in image signals. In other words, the configuration of thedigital camera system and the focus detection operation (correspondingto the flowchart illustrated in FIG. 9) according to the secondexemplary embodiment are similar to those of the first exemplaryembodiment, and the sub-routine in step S1903 according to the secondexemplary embodiment is different from that of the first exemplaryembodiment. FIG. 11 illustrates a flowchart for the sub-routine in stepS1903, according to the second embodiment.

In steps S2101 to S2104, like in steps S2001 to S2004 according to thefirst exemplary embodiment, the first and second filter processing areperformed on a pair of image signals, respectively, to acquire the firstand second correlation amounts. In the present exemplary embodiment, thedegree of effect of noise can be determined without using the first andsecond correlation amounts in step S1903, and thus this step can beomitted.

In step S2105, a contrast evaluation value for evaluating the contrastof a pair of image signals is acquired as information about the contrastof a subject. In the present case, a wave form amplitude PB, which is adifference between a maximum value (peak value) and a minimum value(bottom value) of an image signal, is calculated as one of evaluationvalues. The amplitudes PB of the A image signal and the B image signalare acquired, and the average of the amplitudes is used as an average PBof the pair of image signals. Only the amplitude PB of one of the Aimage signal and the B image signal may be acquired, and the acquiredamplitude may be used as the amplitude PB of the pair of image signals.Further, in step S2105, an evaluation value representing a sharpness ofan image is calculated by the following formula as one of contrastevaluation values.

$\begin{matrix}{{Cnt} = \frac{\overset{n - 1}{\sum\limits_{k = 0}}\left( {a_{k} - a_{k + 1}} \right)^{2}}{\sum\limits_{k = 0}^{n - 1}{{a_{k} - a_{k + 1}}}}} & {{Formula}\mspace{14mu} 1}\end{matrix}$

where a_(k) represents a sub-pixel column signal for focus detection(i.e., A image or B image), and n represents the number of pixels in thesub-pixel column. As an example, FIG. 12 illustrates a relationshipbetween a sharpness and a defocus amount in a high-frequency subject(solid line) and a low-frequency subject (dashed line). FIG. 12illustrates that a subject image is sharpened as the defocus amountdecreases (blurring is reduced), so that the sharpness of the imagesignal increases. This indicates that even when the defocus amount ofthe low-frequency subject becomes smaller, the sharpness of the imagesignal is not increased, compared with the high-frequency subject. Whenthe amount of noise contamination in a pair of image signals is thesame, the effect of noise is reduced as the waveform amplitude PB of theimage signal becomes larger, or as the image Sharpness of the signalbecomes higher.

In the subsequent step S2106, a noise evaluation value for evaluatinginformation about noise contamination in image signals is calculated. Inthis case, image sensor output values such as dark current noise andphoton shot noise, which are noise components of photodiodes, andvarious setting parameters that may be associated with a gain such as anaccumulation time and an ISO sensitivity, which are image pickupconditions, are converted into a table or a formula. A noise evaluationvalue Noise is calculated based on coefficients provided in the table orformula As a simple calculation, the noise evaluation value depending onphoton shot noise and ISO sensitivity is calculated by the followingformula.Noise (Peak, ISO)=√{square root over (Peak×ISOGain)}  Formula 2where ISOGain represents a value indicating the amount of gain accordingto the value of the ISO sensitivity, and Peak represents a maximum valueof an image signal. Although not described in the above formula, forexample, noise components of fixed patterns generated depending on thedifference between the characteristics of vertical readout lines foreach column may be listed in the table. The table may be measured inadvance for each component as values according to an output value of afocus detection signal, to be used in a calculation.

In the subsequent steps S2107 and S2108, the degree of effect of noiseis determined based on the contrast evaluation value and the noiseevaluation value which are acquired in step S2105 and step S2106,respectively. In step S2107, it is determined whether the ratio(Noise/PB) between the noise evaluation value Noise and the wave formamplitude PB of the image signal as one of contrast evaluation values isless than a fifth threshold. If the ratio is less than the fifththreshold (YES in step S2107), the processing proceeds to step S2108. Onthe other hand, if the ratio is equal to or more than the fifththreshold in step S2107 (YES in step S2107), the processing proceeds tostep S2109, and it is determined that the degree of effect of noise onthe correlation calculation is large.

In step S2108, it is determined whether the ratio (Noise/Sharpness)between the noise evaluation value Noise and the image Sharpness of thefocus detection signal is less than a sixth threshold. If the ratio isless than the sixth threshold (YES in step S2108 ), the processingproceeds to step S2110, and it is determined that the degree of effectof noise on the correlation calculation is small. On the other hand, ifit is determined that the ratio is equal to or more than the sixththreshold in step S2108 (NO in step S2108), the processing proceeds tostep S2109, and it is determined that the effect of noise on thecorrelation calculation is large, and thus the sub-routine operation isterminated. If the processing proceeds to step S2109, it is determinedthat the effect of noise on the correlation calculation is large. Then,the sub-routine operation is terminated, and the processing returns tothe flowchart of FIG. 9. If the processing proceeds to step S2109, theprocessing proceeds to step S1905, and if the processing includes stepS2110, the processing proceeds to step S1906. Step S1904 and subsequentsteps are the same as those of the first exemplary embodiment. Based onthe determination result in step S1903, it is selected whether toacquire the phase difference between two images by the correlationcalculation based on the image signals applied to the first or second,filter. However, if the first and second correlation amounts are notacquired by the sub-routine in step S1903, the band-pass filter selectedbased on the determination result in step S1903 is applied to a pair ofimage signals, and the phase difference between two images is acquiredby the correlation calculation using the pair of image signals processedby the selected filter. Alternatively, the first and second, filterprocessing may be performed on the pair of image signals before stepS1903 is finished. In this case, one of the filter applied to the imagesignals is selected to carry out the correlation calculation on thebasis of the determination result in step S1903. In the presentdisclosure and specification, Assume herein that a filter is deemed tobe selected also when the signal applied to different filter isselected.

Through the operations as described above, the degree of effect of noiseon the focus detection can be determined. In addition, by selecting anappropriate filter using the determination result, the focus detectioncan be performed in which the effect of correlation noise is reduced. Inthe first exemplary embodiment, the degree of effect of correlationnoise is determined based on the result of estimating the degree ofcorrelation noise, while in the second exemplary embodiment, the degreeof effect of correlation noise is determined based on the magnitude ofnoise. The first and second exemplary embodiments can be selectivelyused in the digital camera systems depending on the scenes. Each scenemay be discriminated by the user switching a mode.

Modified Example of Second Exemplary Embodiment

A modified example for step S1903 will now be described. According tothe second exemplary embodiment, in step S1903, information about noise(noise evaluation value Noise) and information about a contrast of animage signal (wave form amplitude PB, Sharpness) are acquired, and thedegree of effect of noise is determined based on the magnitude of theratios therebetween (steps S2105 to S2110). Instead of using theindividual methods, the following simple method may be used. That is, athreshold is set to the wave form amplitude PB for each ISO sensitivity.If the wave form amplitude PB is less than the threshold, it isdetermined that the effect of noise is large, and if the wave formamplitude PB is equal to or more than the threshold, it is determinedthat the effect of noise is small. The thresholds for the wave formamplitude PB set for each ISO sensitivity may be converted into a tableand stored. With this method, the degree of effect of noise can bedetermined only with reference to the wave form amplitude PB and afilter can be selected based on the determination result. Even in thecase where the degree of effect of noise is determined by using thethreshold set to any one of information about a contrast and informationabout noise, the degree of effect of noise is deemed to be determined onthe basis of the information about noise and the information about acontrast.

Examples of the information about noise include image sensor outputvalues such as dark current noise and photon shot noise, which are noisecomponents of photodiodes, and an accumulation time and ISO sensitivitywhich are image capturing conditions. The noise evaluation value may beset using a plurality of pieces of information selected from among thepieces of information described above, or may be set using only a singlepiece of information (e.g., ISO sensitivity).

In addition, a filter may be selected depending on informationindicating whether the subject is susceptible to noise. In general, aperson's face or the like is a subject that includes a portion having ahigh spatial frequency and a portion having an extremely low spatialfrequency, and the subject is extremely susceptible to noise in aportion having a low spatial frequency. Accordingly, the focus detectionapparatus including a unit for detecting the position, size, detectionreliability, and the like of a face present within an angle of field inaddition to the configuration according to the present exemplaryembodiment, may select a filter according to face detection informationand image pickup conditions. Specifically, if the ISO sensitivity, whichis an image pickup condition, is equal to or higher than a predeterminedvalue and an area in which a face is detected and a focus detection areaare superimposed on each other, it is determined that the degree ofeffect of noise is large and a filter having a relatively low frequencycharacteristic is selected.

A modified example of the image sensor according to the first and secondexemplary embodiments will be described. As described above withreference to FIG. 3A, the image sensor has a configuration in which eachunit pixel includes a set of the first focus detection pixel S1, thesecond focus detection pixel S2, and the microlens ML and h pixels inthe horizontal direction and v pixels in the vertical direction arearranged. However, the configuration of the image sensor is not limitedto this configuration, and the unit pixels and normal pixels which arenot divided on the exit pupil surface may coexist. Even in thisconfiguration, advantageous effects similar to those described above canbe obtained.

Exemplary embodiments described above are merely representativeexamples. Various modifications and alterations can be made on theexemplary embodiments without departing from the scope of the appendedclaims.

Accordingly, it is possible to provide a focus detection apparatuscapable of performing focus detection while reducing an effect of noise.

Other Embodiments

One or more embodiment(s) of the present disclosure can be realized by acomputer of a system or apparatus that reads out and executes computerexecutable instructions (e.g., one or more programs) recorded on astorage medium (which may also be referred to more fully as a‘non-transitory computer-readable storage medium’) to perform thefunctions of one or more of the above-described embodiment(s) and/orthat includes one or more circuits (e.g., application specificintegrated circuit (ASIC)) for performing the functions of one or moreof the above-described embodiment(s), and by a method performed by thecomputer of the system or apparatus by, for example, reading out andexecuting the computer executable instructions from the storage mediumto perform the functions of one or more of the above-describedembodiment(s) and/or controlling the one or more circuits to perform thefunctions of one or more of the above-described embodiment(s). Thecomputer may comprise one or more processors (e.g., central processingunit (CPU), micro processing unit (MPU)) and may include a network ofseparate computers or separate processors to read out and execute thecomputer executable instructions. The computer executable instructionsmay be provided to the computer, for example, from a network or thestorage medium. The storage medium may include, for example, one or moreof a hard disk, a random-access memory (RAM), a read only memory (ROM),a storage of distributed computing systems, an optical disk (such as acompact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™),a flash memory device, a memory card, and the like.

While the present disclosure has been described with reference toexemplary embodiments, it is to be understood that the inventiveconcepts described herein are not limited to the disclosed exemplaryembodiments. The scope of the following claims is to be accorded thebroadest interpretation so as to encompass all modifications andequivalent structures and functions.

This application claims the benefit of Japanese Patent Application No.2017-025380, filed Feb. 14, 2017, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. A focus detection apparatus comprising: adetermination unit configured to determine a degree of effect of noiseincluded in a pair of parallax image signals; and an acquisition unitconfigured to acquire information about a phase difference between thepair of parallax image signals based on a calculation of correlationbetween the pair of parallax image signals, wherein the acquisition unitselects, based on a determination result of the determination unit, afilter used to acquire the information about the phase difference fromamong a plurality of filters having different frequency characteristics,and outputs, as a focus detection result, the information about thephase difference acquired by the correlation calculation based on thepair of parallax image signals applied to the selected filter.
 2. Thefocus detection apparatus according to claim 1, wherein, when thedetermination unit determines that the degree of effect of noise isequal to or greater than a predetermined degree, the acquisition unitselects a first filter as a filter used to acquire information about animage shift amount, and when the determination unit determines that thedegree of effect of noise is less than the predetermined degree, theacquisition unit selects a second filter as the filter used to acquirethe information about the image shift amount, wherein the second filteris configured to transmit frequency components lower than frequencycomponents transmitted by the first filter.
 3. The focus detectionapparatus according to claim 2, wherein the determination unitdetermines the degree of effect of noise based on a difference between athird correlation amount acquired by a correlation calculation based onthe pair of parallax image signals using a third filter and a fourthcorrelation amount acquired by a correlation calculation based on thepair of parallax image signals using a fourth filter configured totransmit frequency components lower than frequency componentstransmitted by the third filter.
 4. The focus detection apparatusaccording to claim 2, wherein the determination unit determines thedegree of effect of noise based on a difference between a firstcorrelation amount acquired by a correlation calculation based on thepair of parallax image signals using the first filter and a secondcorrelation amount acquired by a correlation calculation based on thepair of parallax image signals using the second filter.
 5. The focusdetection apparatus according to claim 3 wherein the determination unituses information about an intensity of at least one of signalsconstituting the pair of parallax image signals, to determine the degreeof effect of noise.
 6. The focus detection apparatus according to claim5, wherein the information about the intensity of the at least one ofsignals is at least one of a maximum value of the intensity of thesignals and an average value of the intensity of the signals.
 7. Thefocus detection apparatus according to claim 1, wherein thedetermination unit determines the degree of effect of noise included inthe pair of parallax image signals based on information about a contrastof the pair of parallax image signals and information about noise. 8.The focus detection apparatus according to claim 1, wherein theinformation about the contrast is at least one of an amplitude and asharpness of at least one of image signals constituting the pair ofparallax image signals.
 9. The focus detection apparatus according toclaim 7, wherein the information about the noise is at least one of anoise component caused by an image sensor used to acquire the pair ofparallax image signals and a set value for a gain component applied tothe pair of parallax image signals.
 10. The focus detection apparatusaccording to claim 9, wherein the set value for the gain component is atleast one of a sensitivity and an accumulation time for accumulatingelectric charges when the pair of parallax image signals is acquired.11. The focus detection apparatus according to claim 7, wherein thedetermination unit determines the degree of effect of noise by comparingan evaluation value acquired based on the information about the contrastof the pair of parallax image signals with a threshold determined basedon the information about the noise in the pair of parallax imagesignals.
 12. The focus detection apparatus according to claim 11,wherein when the evaluation value is greater than the threshold, thedetermination unit determines that the degree of effect of noise isequal to or lower than a predetermined value, and when the evaluationvalue is equal to or less than the threshold, the determination unitdetermines that the degree of effect of noise is higher than thepredetermined value.
 13. The focus detection apparatus according toclaim 1, wherein the acquisition unit acquires the pair of parallaximage signals applied to the selected filter, by performing filterprocessing using the selected filter on the pair of parallax imagesignals.
 14. The focus detection apparatus according to claim 1, whereinthe acquisition unit acquires a plurality of pairs of parallax imagesignals applied to a plurality of filters, by performing filterprocessing using the plurality of filters on the pair of parallax imagesignals, and acquires information about the image shift amount byselecting the pair of parallax image signals applied to the selectedfilter from among the plurality of pairs of parallax image signals andcarrying out the correlation calculation based on the selected pair ofparallax image signals.
 15. The focus detection apparatus according toclaim 1, wherein the acquisition unit acquires a plurality of pairs ofparallax image signals applied to a plurality of filters, by performingfilter processing using the plurality of filters on the pair of parallaximage signals, the acquisition unit acquires a plurality of pieces ofinformation about the image shift amount by correlation calculationsbased on the plurality of pairs of parallax image signals, and theacquisition unit acquires information about the image shift amount byselecting information about the image shift amount by a correlationcalculation based on the pair of parallax image signals applied to theselected filter from among the plurality of pieces of information aboutthe image shift amount.
 16. The focus detection apparatus according toclaim 1, wherein the determination unit uses sensitivity for imagecapturing to acquire the pair of parallax image signals to determine thedegree of effect of noise.
 17. The focus detection apparatus accordingto claim 1, wherein the determination unit uses an exposure value (EV)for image capturing to acquire the pair of parallax image signals todetermine the degree of effect of noise.
 18. The focus detectionapparatus according to claim 1, wherein the pair of parallax imagesignals is based on a signal generated by photoelectrically convertinglight from an image pickup optical system.
 19. The focus detectionapparatus according to claim 1, wherein the information about the phasedifference is at least one of a defocus amount and a drive amountnecessary for causing a lens included in the image pickup optical systemto move to an in-focus position.
 20. An image pickup apparatuscomprising: an image sensor configured to obtain a pair of parallaximage signals by photoelectrically converting light from an image pickupoptical system; and a focus detection apparatus comprising: adetermination unit configured to determine a degree of effect of noiseincluded in the pair of parallax image signals; and an acquisition unitconfigured to acquire information about a phase difference between thepair of parallax image signals based on a calculation of correlationbetween the pair of parallax image signals, wherein the acquisition unitselects, based on a determination result of the determination unit, afilter used to acquire the information about the phase difference fromamong a plurality of filters having different frequency characteristics,and outputs, as a focus detection result, the information about thephase difference acquired by the correlation calculation based on thepair of parallax image signals applied to the selected filter.